pH glass electrodes exist which are subjected to pressure during the production process in the reference and/or bridge electrolyte chamber. This method is used to prevent a continuous discharge of reference and/or bridge electrolyte, and contamination of the electrode of measuring medium, during use.
During the production process, these electrodes are stored in a pressure tank at a pressure of, for example, 10 bar. An elevated inner electrode pressure accumulates after a time via the porous diaphragm of the electrode. After a certain time, the sensors are removed from the pressure tank, and the inner electrode pressure is checked. The fill-level of the set pressure is frequently evaluated using an air bubble that is a few millimeters in size. This air bubble is embedded in the sensor by design. If a nominal pressure of, for example, 4 bar is not reached, the electrode is again stored in the pressure tank under an overpressure for a specific period.
Since the pressure setting differs from sensor to sensor, there is no specific number of storage cycles in the pressure tank. Continuously monitoring the inner electrode pressure and the associated quality of the measurement is impossible given the current state of the art.
During production and in industrial use, the actual amount of the pressure in the electrode cannot be precisely determined. Normally, this is assessed on the basis of the size of the air bubble in the interior of the electrode. In the process, electrodes may be subject to insufficient or excessive pressure. When using the electrode, a reduction in pressure occurs from the bridge and/or reference electrolyte being pressed out through the diaphragm. The rate of pressure reduction depends upon numerous factors, such as the temperature, process pressure, and porosity of the diaphragm. If the process pressure exceeds the inner pressure of the electrolyte, the reference electrolyte becomes contaminated; frequently, this can be detected only by a drift in the measured values.